Hot deformation behavior of an aluminum-matrix hybrid nanocomposite fabricated by friction stir processing

A fine-grained aluminum-matrix hybrid nanocomposite reinforced with TiO2, MgO and Al3Ti nanoparticles was prepared via reactive friction stir processing (FSP) of an Al-Mg sheet with pre-placed TiO2 particles (50 nm; 3.1 vol%). The microstructure of the hybrid nanocomposite comprises high-angle grain boundaries (~90%) with an average size of 2 µm and hard inclusions with sizes in the range of 30-50 nm. Evaluation of the hot deformation behavior of the nanocomposite by uniaxial tensile testing at different temperatures (300-450 °C) and strain rates (0.001-0.1 s−1) shows that the deformation apparent activation energy of the nanocomposite is 137 kJ mol−1 at ≤300 °C. The values of the activation energy for the Al-Mg alloy before and after FSP at this temperature range are about 105 and 135 kJ mol−1, respectively. This observation highlights the role of ultrafine hard particles and the structural changes induced by FSP on the deformation process. At the higher temperatures, the deformation activation energy for the aluminum alloy without and with the reinforcing particles is 303 and 456 kJ mol−1, respectively. Detailed microstructural analysis by electron back scattered diffraction and transmission electron microscopy suggests that dynamic recrystallization is responsible for the deformation behavior at the elevated temperatures. Meanwhile, the presence of the hard nanoparticles operates as a grain growth inhibitor improving the thermal stability of the fine-grained aluminum alloy.